Anode for the electrowinning of manganese



United States Patent() 3,301,777 ANODE FOR THE ELECTRWINNING F MANGANESE Robert E. Leonard, Henderson, Nev., assignor to American Potash & Chemical Corporation, Los Angeles,

Calif., a corporation of Delaware Filed Apr. 1, 1963, Ser. No. 269,497 1 Claim. (Cl. 204-290) This invention relates to the production of electrolytic maganese metal. More specifically, this invention relates to the use and construction of anodes in the electrolytic production of manganese metal.

Considerable difficulty has been experienced in the electrowinning of manganese metal from aqueous manganesebearing solutions due to excessive corrosion at the solution line of the anode used in the electrowinning process. Various solutions to this problem have been proposed, including, for example, the use of special anode cornpositions, special cell operating conditions and the like, but these proposed solutions have been generally unsuccessful.

These difficulties are overcome in accordance with the present invention by preparing and utilizing an anode having a generally circular cross-section at the solution line and protected at the solution line by an inert protective covering possessed of relatively high tensile strength.

The term generally circular as used in this specification and in the appended claim refers to all smoothly contoured cross-sections having only convex exterior surfaces, and includes not only circular cross-sections, but also elliptical cross-sections.

The cross-sectional configuration of the anode outside of the immediate area of the solution lineis not critical and can be adjusted to any geometric configuration desired. Generally, the anodes of this invention have an elongated body configuration adapted to extend substantially the full depth of an electrolytic cell.

For a more complete understanding of this invention reference is made to the following description taken in conjunction with the accompanying drawing in which:

FIG. l is a cross-sectional view through the solution line portion of an anode of this invention taken on line 1-1 of FIG. 3.

FIG. 2 is a cross-sectional view through the solution line portion of another embodiment of this invention taken on line 2-2 of FIG. 4.

FIG. 3 is a perspective view of an anode of this invention.

FIG. 4 is a perspective view of another anode embodirnent of this invention having an elliptical configuration at the solution line portion tapering to a generally flat and broadened cross-section at the lower extremity of the elongated anode body. l v

As shown on the drawings, the anodes 18 and 20 are provided at the solution line portion thereof with a generally circular cross-section 10, and a tightly adherent, inert protective covering 12, of high tensile strength.

In FIG. 2, cross-section 16 is taken through section 2-2 of FIG. 4. Likewise, cross-section 14 in FIG. l is taken through section 1*-1 of FIG. 3.

The anode materials utilized in practicing the present invention are those generally employed in the art, including, for example, anodes composed of graphite, lead and lead alloys, such as lead-silver, lead-arsenic, leadbismuth, lead-antimony alloys, noble metals, noble metal alloys, lead dioxide, various combinations of one or more clad or plated materials, and the like.

I 3,301,777 Patented Jan. 31, 1967 While this invention is not limited to any theory, it is believed that solution line corrosion occurs on an anode which is covered at the solution line with some protective covering by the following mechanism. A slight corrosion of the alloy at the bottom of the coating below the solution line will open up a small crack by minutely undercutting the coating. Due to the nature of the electrolytic process, a small amount of manganese dioxide is deposited at the anode while manganese metal is being plated out at the anode. As it forms, this manganese dioxide will project itself into the small crevice opened by the slight corrosion. This wedging action of the manganese dioxide places the protective coating under tension, thus causing the coating to spall off, particularly if the coating is weak, under tension or is applied to a long, flat surface where the mechanical advantage of the coating is poor.

According to the present invention, this wedging action and the resultant corrosion are prevented by the use of anodes having generally circular cross-sections at the solution line and covered at the solution line by a protective coating having a relatively high tensile strength.

Because of the geometry of the anode at the solution line, tight adherence to the anode by the protective coating is not a critical factor. Thus, perfect adhesion at all points of contact between the coating of the anode is not necessary. It is only necessary to select a coating which has resistance to the anolyte, greater than that of the anode itself, and sufficient tensile strength to withstand the wedging action of the electrolytically deposited manganese dioxide.

The geometry of the anode is such that it is impossible for the manganese dioxide to achieve great enough leverage to strip the protective coating from the generally circular anode.

The inert protective coating, because of its high tensile strength and the geometry of the anode, withstands the wedging action of the manganese dioxide under the edge of the protective coating. Since some undercutting of the protective coating, as described above, will inevitably occur and some manganese dioxide will wedge underneath the edge of the coating, the result is that the tightly wedged manganese dioxide effectively seals the undersolution edge of the coating and prevents further attack.

The width of the protective coating on the anode is not critical so long as it adequately covers that portion of the anode that is in immediate contact with the surface of the electrolytic solution. Preferably, adequate covering is provided to compensate for any fluctuation in the electrolyte level, so that the protective coating may extend along the longitudinal axis of the electrode for a distance of from as little as about 1A inch or less, up to 6 inches, or even more.

The material from which the protective coating is prepared should be possessed of sufficient 4tensile strength to resist the wedging action of the manganese dioxide, as discussed above, and should also be inert to the environment in which it is placed. The protective coating may be provided with the required tensile strength, either through the inclusion of fillers, such as glass fibers, filaments or cloth, or the tensile strength of the material itself -rnay be such that such fillers are not necessary.

The tensile strength Aof the protective coating should be at least about 25,000 pounds per square inch and preferably at least about 40,000 pounds per square inch.

Suitable binder materials for the preparation of the protective coatings of this invention are substantially linert to an acidic oxidizing environment and include, for

-along with otherwise identical uncoated anodes.

d example, polyesters, epoxies, resorcinol-formaldehyde, polycarbonates, polyvinyl chlorines, fluorocarbons, and the like. Other suitable binders are listed in the Modern iPlastics Encyclopedia, Sept. 1962, vol. 40 No. lA.

Suitable iller materials for use with the binder materials in the protective coatings of this invention are substantially inert to an acidic oxidizing environment, and should generally be high tensile strength materials. Filler materials include, for example, the iber or filament forms of soft or hard glass, aluminum silicate, other ceramics, asbestos, fused quartz and the like. These filler materials can be arranged and utilized in any form, as desired, including for example, continuous laments, spunbers, mattes, woven fabrics and the like. Titanium, tantalum and other metals which form stable anodic lms can be used as filler materials in the form of wires, woven screens, expanded metal mesh and the like.

In the specification, claim and following specic examples, all parts and percentages are by weight unless otherwise indicated. The following examples are set forth to further illustrate and not to limit the invention.

EXAMPLE I An anode is fabricated, using 0.150 inch diameter rods of 99% lead-1% silver alloy. Two of the four rods are coated at the solution line with a 1/16 inch thick layer of epoxy resin reinforced with glass fibers. The other two rods are not protected from solution line corrosion in any way.

The four rods are inserted into a manganese metal electrowinning cell. The cell containing four rods is operated for 3,180 ampere hours days) with 70 to 80 ampere per square foot current density at the anode. Marked solution line corrosion occurs on the two rods which are unprotected at the `solution line. The solution line diameter of these rods is reduced to 0.085 inch at the narrowest point. No `attack occurs at the solution line of the two rods which were provided with the protective coating at the solution line.

At the end of this ten day operating period, the resin coating is cut from the two protected rods longitudinally and the alloy underneath examined. There is no visible evidence of corrosion on the portion of the rods covered by the protective coating.

EXAMPLE II A 94% lead-6% antimony alloy anode having a 3A inch diameter is coated at the solution line with a layer of polyester resin reinforced with glass filaments. This anode, carrying the protective coating at the solution line, is immersed in a manganese electrowinning cell This cell is operated continuously for 16 days, after which time `anodes are inspected for any signs of corrosion. On the uncoated anodes, solution line corrosion is evident. This corrosion is about f/l@ of an inch wide and about 1/16 of an inch deep at the solution line. The anode carrying the protective coating at the solution line discloses no visible evidence of corrosion at the solution line; also there is no evidence of undercutting taking place at the under-solution junction of the coating and the anode. The polyester coating has a hard surface and appears to be in good condition.

Analogous results are obtained using plastic-impregnated pressure sensitive tape as the protective coating on the anode.

In general, the electrolytic production of manganese involves the steps of roasting and 4acid leaching a manganese ore to produce a solution of dissolved manganese which is then purified in preparation for electrolysis. The puried manganese solution, which may or may not include additives such as copper or sulfur compounds, is then subjected to electrolysis in an appropriate electrolytic cell. Manganese metal is deposited at the cathode. After a coating of manganese metal of desired thickness has been deposited, the cathode containing the deposit is removed from the cell and the manganese is removed by appropriate means.

Typical manganese metal stripping procedures include, for example, physical impact, thermal shock, ultrasonic treatment of the plated cathode, and the like. The stripped cathode can be returned for further use in the cell.

The electrolytic cells utilized in the process of this invention can be any of those which are conventional in the art including, for example, continuous-belt cathode cells, diaphragm cells, diaphragmless common electrolyte cells, and the like.

Conventional manganese metal electrowinning cells generally contain a plurality of anodes and cathodes separated from one another by porous diaphragms. The feed solution flows into contact with the cathode and the spent electrolyte is discharged from the anode compartrnent. The cell configuration and materials of construction, except for the anode, form no part of this invention. This invention can be applied to any of the conventional cell configurations known to those skilled in the art.

The process of this invention is generally applicable to the preparation of electrolytic manganese using both chloride and sulfate electrolytes. The electrolytes utilized inthe present invention can include any of the conventional electrolyte additives such as: ammonium cornpounds, reducible sulfur compounds, trace quantities of soluble copper and the like. The process of this invention is applicable to the production of the gamma, alpha and other forms of manganese metal.

When the present invention is utilized in the sulfate process for the production of electrolytic manganese according to one preferred procedure, the operating variables can be conveniently adjusted within the following limits. The concentration of manganese sulfate in the electr-olyte can be adjusted to between about 8 to 70 grams per liter. If desired, between about 0.05 and 1.0 gram per liter of a reducible sulfur compound such as dithionates, xanthate, suliides, sulfites or sulfur dioxide can be included in the electrolyte. Preferably, from about 0.1 to about 0.5 gram per liter of SO2 are present. The catholyte temperature can be regulated from about 20 C. to about 60 C., preferably between about 35 C. and 40 C. From about 90 grams per liter to a quantity sufficient to saturate the electrolyte of a buffer such as (NH4)2SO4 can be included in the electrolyte if desired. Preferably (NI-14h80., is present in an amount ranging from about 120 to 160 grams per liter.

Conditions similar to those described herein with respect to-the sulfate electrolyte can be utilized when operating the manganese producing process of this invention with a chloride electrolyte except that chloride compounds replace the sulfate compounds.

While the general operating variables have 'been described for both the chloride and sulfate processes, this invention is not limited to any specific operating variables.

The conventional cell anolyte can contain from g./l. to the saturation point (preferably -l60 grams per liter) of ammonium sulfate; from 3-25 g. Mn/l. (preferably from 11-13 g. Mn/l. as MnSO4) and from 20-65 g./1. (preferably 35-40 g./l.) of H2804.

The anodes, in accordance with this invention, enjoy considerably extended operating lives because they are not corroded into uselessness at the solution line before under-solution corrosion destroys .the usefulness of the anode.

As will be understood by those skilled in the art, what has been described are the preferred embodiments of the invention. However, many modications, changes, and substitutions can be made therein without departing from the scope and spirit as defined in the following claim.

What is claimed is:

In an anode adapted for immersion in aqueous manganese-bearing solutions for the electrowinning of manganese metal having a` generally circular cross-sectional conguration at the solution line and a protective covering at the solution line, the improvement which consists essentially in forming said protective covering of an inert organic binder material selected from the group consisting of polyesters, epoxies, resorcinol-formaldehyde, polycarbonates, polyvinyl chlorides and uorocarbons, said binder being filled with fibers selected from the group consisting of glass, aluminum silicate, asbestos and fused quartz.

References Cited by the Examiner UNITED STATES PATENTS Blackman 204-290 X Levett 204-290 X Dyer 204-290 Ames 204-279 X Godsey 204-286 Carosella 204-105 Carosella 204-105 JOHN H. MACK, Primary Examiner.

D. R. JORDAN, Assistant Examiner.A 

